The present invention relates to a system and method for determining the vibration frequency of an electroded crystal plate, preferably carried out during the design of a piezoelectric device incorporating the electroded crystal plate and before production thereof. This invention also relates to a piezoelectric device that has been designed in accordance with the method of the invention. The invention further relates to a program of instructions for carrying out the design-based method of the invention.
A wide variety of piezoelectric devices are used in various electronic applications. One common type of piezoelectric device is a crystal resonator. A typical crystal resonator includes a layer of crystalline piezoelectric material having opposite faces, each having a corresponding electrode bonded thereto to sandwich the piezoelectric material between the electrodes. The crystal resonator vibrates in response to an electrical stimulus applied to the electrodes. The vibration induces a highly stable electrical oscillation across the electrodes that is useful for timing other devices.
For a piezoelectric device to operate properly, it is important for its vibration frequency and displacement characteristics to fall within design specifications. For example, if these characteristics fall outside design specifications, the piezoelectric device may not have the desired oscillation frequency or magnitude response. Unfortunately, it has proven very difficult to precisely determine such elastic properties of piezoelectric devices. One reason for this difficulty is that there is considerable interplay between the various elastic properties of a piezoelectric material. This difficulty is compounded when other components of the device, such as the electrodes, are also considered.
Due to such difficulties, piezoelectric devices generally are formed in a rough state that is not guaranteed to be within final design specifications. The piezoelectric devices may then be brought into final design specifications by adding or removing material from the piezoelectric device. In one conventional approach, material is added or removed from electrodes. In another conventional approach, stiffening electrical fields are applied to a piezoelectric device during operation. In a third conventional approach, a piezoelectric device is stiffened to reduce acceleration sensitivity by adding one or more braces either on the electrodes or on the layer of piezoelectric material.
Such conventional approaches to providing piezoelectric devices with desired elastic properties have several drawbacks. They are not truly design based, but rather require extra fabrication steps, such as adding or removing material from electrodes, or special operating environments, such as appropriate stiffening electrical fields. Generation of stiffening electrical fields may require additional circuitry. Conventional approaches typically also require the formation of various prototype devices to determine how to fabricate the piezoelectric device with a suitable rough state as described above. Furthermore, conventional approaches are believed to work poorly where electrode thickness exceeds about two percent of total device thickness.
Patent application Ser. No. 09/212,816, entitled xe2x80x9cStiffness Effects in Piezoelectric Devices,xe2x80x9d filed Dec. 16, 1998, provides a design-based system and method for verifying designs of a piezoelectric device during the design process and before any manufacturing steps are carried out. In particular, the system and method verifies whether the electrodes have the appropriate stiffness characteristics. With this arrangement, extra fabrication steps and generation of special operating environments are not necessary. The application also provides an improved piezoelectric device that meets final design specifications while reducing the need for post-production processing of the device.
Patent application Ser. No. 09/333,721, entitled xe2x80x9cCorrection Factors for the Analysis of Piezoelectric Devices,xe2x80x9d filed Jun. 15, 1999, provides a system and method for determining and applying correction factors in the analysis of the vibration frequency and displacement characteristics of the piezoelectric material itself. This analysis is also carried out during the design and before production of the piezoelectric device.
It is an object of the present invention to provide further improvements to the design-based system and method for analyzing piezoelectric devices.
It is another object of this invention to provide a system and method for determining the vibration frequency and displacement characteristics of the piezoelectric crystal material and the attached electrodes, considered as a single unit, during the design of a piezoelectric device incorporating these components and before production thereof.
It is a further object of this invention to provide a piezoelectric device that has been designed in accordance with the method of the invention.
In one aspect, the invention involves a method for verifying a design of an electroded crystal plate, preferably made of AT-cut crystal. The method comprises forming a model, including one or more correction factors, of the electroded crystal plate, and analyzing the model to determine one or more vibration frequency characteristics of the electroded crystal plate to determine whether it meets a design specification. The vibration frequency characteristics determined may include the fundamental thickness-shear frequency, coupling characteristics between the thickness-shear vibration mode and at least one other vibration mode such as the flexural mode, or coupling characteristics between the thickness-stretch vibration mode and at least one other vibration mode such as the flexural mode.
The one or more correction factors are developed by forming a stress-strain model of the electroded crystal plate, forming a motion model of the electroded crystal plate based on the formulated stress-strain model, and forming a frequency model of the electroded crystal plate based on the formulated stress-strain model and the motion models.
Another aspect of the invention involves an apparatus, such as a computer system, for verifying the design of an electroded crystal plate or a piezoelectric device incorporating such a plate. The apparatus comprises the appropriate hardware or software for carrying out the above-described method.
In another aspect, the invention involves a processor-readable medium embodying a program of instructions, e.g., software, for execution by the processor for performing the design verification method described above.